Photovoltaic solar cells generally include a first doped semiconductor that forms a junction with a second semiconductor. The first and second semiconductors are oppositely doped, so one is p doped and the other is n doped. The first and second semiconductors can be the same compounds or element which form a homojunction, or can be different semiconductors which form a heterojunction. The heterojunction devices have layers with different band gaps, and therefore different band pass characteristics, to enable the photovoltaic cell to be responsive only to certain wavelengths from an optical source.
One particularly advantageous type of photovoltaic cell, particularly adapted for solar cell applications, includes semiconductors of the Group II-VI type, particularly CdTe. Cadmium telluride-cadmium sulphide heterojunctions have been found to be particularly advantageous because CdS has a much wider band gap than CdTe, thereby enabling coupling of wide band optical energy to the junction. Such heterojunctions require the cadmium telluride to be p doped because cadmium sulphide can only be n doped. Homojunction cadmium telluride photovoltaic cells, i.e., cells wherein the junction is formed by oppositely doped CdTe regions, are also advantageous.
Cadmium telluride photovoltaic cells can take many forms. In one form, the cadmium telluride forms a substrate on which is deposited cadmium sulphide to form a mesa junction or on which cadmium sulphide is epitaxially deposited. In a second form, the cadmium telluride is a thin film coated on a ceramic substrate and the cadmium sulphide is a thin film that is coated, in some instances in an epitaxial manner, on the cadmium telluride. Alternatively, the cadmium sulphide is deposited on an optically transparent, glass substrate and a cadmium telluride film is formed on the cadmium sulphide deposit, e.g. by vapor deposition. In another configuration, the cadmium telluride is coated on a metal electrode block and cadmium sulphide is deposited on the cadmium telluride.
One problem with all of the prior art forms of cadmium telluride photovoltaic solar cells is that metal electrodes make poor ohmic contact with the cadmium telluride. The contact problem has been sufficiently great to materially reduce the yield and stability of previously fabricated cadmium telluride solar cells; thus contacts on previously manufactured CdTe solar cells have had a tendency to deterioriate with time. The exploitation of cadmium telluride solar cells has thus been reduced, even though such cells exhibit a relatively high solar efficiency of 10 percent, before contact degradation.
It is, accordingly, an object of the present invention to provide a new and improved cadmium telluride photovoltaic cell.
Another object of the invention is to provide a new and improved cadmium telluride photovoltaic cell having improved ohmic contact characteristics.
Still another object of the present invention is to provide a new and improved cadmium telluride-cadmium sulphide photovoltaic solar cell having improved ohmic contact characteristics.
A further object of the present invention is to provide a new and improved cadmium telluride homojunction photovoltaic cell having improved ohmic contact characteristics.